One we did in grade school was to just stand in a doorway and lift your arms sideways until your wrists touch the doorway. Press as hard as you can to try to lift your arms for a while, then step out of the doorway and relax your arms completely. They seem to rise of their own accord.
Supposedly you can taste hot sauce if you put it on the inside of your wrist. Don't like dump a whole bunch on there, of course.
George Stratton, in the 1890's, made a pair of glasses that inverted everything. After wearing them a while, his brain adapted and everything looked normal. I can't find many references, but I remember hearing that when he removed the glasses after several days he saw everything upside down.
I once wore an eyepatch for a day (no injuries, just wanted to be a pirate), and when I took it off I noticed the room getting darker and brighter as my eyes tried to equalize. When I looked in the mirror, I could see one pupil dilating while the other contracted, and vice versa for a few seconds.
You can also condition your pupils. If you shine a light on your eye (with caution) and play a certain sound at the same time, over and over again, then look in a mirror and play the sound without the light, your pupils will contract.
And supposedly, if you spend your entire day reading stuff online, your butt will eventually assume the shape of a chair...